The conventional communication methods will be explained below. For example, in the wide band CDMA (W-CDMA: Code Division Multiple Access) using the SS (Spread Spectrum) system, turbo codes have been proposed as error-correction codes that greatly exceed convolutional codes in their performances. In the turbo code, a list formed by interleaving an information list is encoded in parallel with a known coding list, and the turbo code is one of the error-correction codes that have attracted the greatest public attention at present, and is said to provide characteristics close to Shannon limit. In the above-mentioned W-CDMA, since the performances of the error-correction code give great effects on the transmission characteristics in the voice transmission and data transmission, the application of the turbo code makes it possible to greatly improve the transmission characteristics.
Operation of transmitting and receiving systems of the conventional communication device using the turbo code will be explained in detail below. FIG. 8 is a drawing that shows the construction of a turbo encoder used in the transmitting system. In FIG. 8(a), reference numeral 101 denotes a first recursive system convolutional encoder that subjects an information list to a convolutional encoding process to output redundant bits. Reference numeral 102 denotes an interleaver, and reference numeral 103 denotes a second recursive system convolutional encoder that subjects the information list that has been switched by the interleaver 102 to a convolutional encoding process to output redundant bits. FIG. 8(b) is a drawing that shows the inner structures of the first recursive system convolutional encoder 101 and the second recursive system convolutional encoder 103, and the two recursive system convolutional encoders are encoders that only output redundant bit respectively. Moreover, the interleaver 102, which is used in the turbo encoder, randomly switches information bit lists.
The turbo encoder, which is arranged as described above, simultaneously outputs an information bit list: x1, a redundant bit list: x2 obtained by encoding the information bit list through the operation of the first recursive system convolutional encoder 101, and a redundant bit list: x3 obtained by encoding the information bit list that has been interleaved through the operation of the second recursive system convolutional encoder 103.
FIG. 9 is a drawing that shows the construction of the turbo decoder that is used in the receiving system. Reference numeral 111 denotes a first decoder that calculates a logarithm likelihood ratio from a receiving signal: y1 and a receiving signal: y2. Reference numerals 112 and 116 denote adders, and reference numeral 113 and 114 denote interleavers. Reference numeral 115 denotes a second decoder that calculates a logarithm likelihood ratio from a receiving signal: y1 and a receiving signal: y3. Reference numeral 117 denotes a deinterleaver, and reference numeral 118 denotes a judging device for judging the output of the second decoder 115, to output an estimated value of the original information bit list. The receiving signals: y1, y2, y3 are signals that are formed by allowing the information bit list: x1 and the redundant bit lists: x2, x3 to include influences from noise and phasing in the transmission path.
In the turbo decoder that is arranged as described above, first, the first decoder 111 calculates the logarithm likelihood ratio: L(Uk) (where k refers to the time) from a received signal: y1k and a received signal: y2k. In this case, the logarithm likelihood ratio: L(Uk) is represented by the following equation:                                                                                              L                  ⁡                                      (                                          u                      k                                        )                                                  =                                                      y                                          1                      ⁢                      k                                                        +                                      La                    ⁡                                          (                                              u                        k                                            )                                                        +                                      Le                    ⁡                                          (                                              u                        k                                            )                                                                                                                                              =                                  Ln                  ⁢                                                                           ⁢                                                            Pr                      (                                                                        x                                                      1                            ⁢                                                          k                              ′                                                                                                      =                                                  1                          ⁢                                                                                                                {                              Y                              }                                                        )                                                                                                                                      Pr                      (                                                                        x                                                      1                            ⁢                                                          k                              ′                                                                                                      =                                                  0                          ⁢                                                                                                                {                              Y                              }                                                        )                                                                                                                                                                                                        (          1          )                    
Here, Le(Uk) represents external information, La(Uk) represents preliminary information that is external information preceding by one, Pr(x1k′=1|{Y}) represents the probability of an estimated information bit upon receipt of the entire list {Y} of the received signals: x1k′ being 1 and, Pr(x1k′=0|{Y}) represents the probability of an estimated information bit upon receipt of the entire list {Y} of the received signals: x1k′ being 0. In other words, equation (1) finds the probability of the estimated information bit: x1k′ becoming 1 with respect to the probability of the estimated information bit: x1k′ being 0.
Next, the adder 112 calculates external information to be given to the second decoder 115 from a logarithm likelihood ratio that is the result of the above-mentioned calculation. Based upon the above-mentioned equation (1), the external information: Le(Uk) is represented by the following equation:Le(Uk)=L(Uk)−y1k−La(Uk)  (2)
Since no preliminary information has been given at the time of the frist decodig process, La(Uk)=0, for an initial time value (k)=0.
In the interleavers 113 and 114, in order to make the received signal: y1k and the external information: Le(Uk) coincident with the time of the received signal:y3, the signals are re-arranged. Then, in the same manner as the first encoder 111, based upon the received signal: y1 and the received signal: y3 as well and the external information: Le(Uk) preliminarily calculated, the second decoder 115 calculates a logarithm likelihood ratio: L(Uk). Thereafter, in the same manner as the adder 112, the adder 116 calculates the external information Le(Uk) by using equation (2). At this time, the external information, rearranged by the deinterleaver 117, is fed back to the first decoder 111 and the preliminary information: La(Uk).
Finally, in the turbo decoder, the above-mentioned processes are repeatedly executed predetermined times so that it is possible to calculate a logarithm likelihood ratio with higher precision, and the judgment device 118 makes a judgment based upon this logarithm likelihood ratio, thereby estimating the bit list of the original information. More specifically, for example, the logarithm likelihood ratio shows that “L(Uk)>0”, the estimated information bit: x1k′ is judged as 1, while it shows that “L(Uk)≦0”, the estimated information bit: x1k′ is judged as 0.
In this manner, in the conventional communication method, by using the turbo code as the error-correction code, even when the signal point-to-point distance becomes closer as the modulation system is multi-valued, it becomes possible to greatly improve the transmitting characteristics in the voice transmission and data transmission, and consequently to obtain characteristics superior to the known convolutional codes.
However, in the above-mentioned conventional communication method, in order to carry out an error correction with high precision, the turbo encoding process is carried out on all the information lists on the transmitting side, and on the receiving side, all the encoded signals are decoded, and a soft-judgment is then executed thereon. More specifically, for example, in the case of 16 QAM, a judgment is made with respect to all the 4-bit data (0000 to 1111: 4-bit constellation), and in the case of 256 QAM, a judgment is made with respect to all the 8-bit data. Therefore, conventionally, the application of the conventional communication method that carries out judgments on all the data as described above causes a problem of an increase in the amount of calculations in the encoder and decoder in response to the multi-valued levels.
Therefore, the object of the present invention is to provide a communication device and a communication method for such a device, which is applicable to any communication system using the multi-carrier modem system and the single-carrier modem system, and makes it possible to achieve a reduction in the amount of calculations and to provide a good transmitting characteristics in the same manner as the conventional device, even when there is an increase in the constellation due to multi-valued levels.